Japanese Patent Publication No. 6-76979 (published on Mar. 18, 1994) discloses an electronic ballast capable of dimming a discharge lamp. The ballast includes a DC voltage regulator providing a variable DC voltage and an inverter converting the DC voltage into a high frequency AC power which is applied through a resonance circuit to the discharge lamp. The DC voltage regulator is controlled by a voltage command to regulate or vary the DC voltage being applied to the inverter, while the inverter is controlled by a frequency command to vary the AC power being applied to the discharge lamp. The frequency command designates a frequency at which the inverter is driven to oscillate for generating the AC power. Also included in the ballast is a dimming controller which, in response to an external dimming signal, provides the voltage command as well as the frequency command for regulating the DC voltage output from the DC voltage regulator and the AC power output from the inverter for dimming the lamp. The dimming controller is configured to periodically change the frequency between two different values in order to operate the lamp stably in a low light output range. For this purpose, the ballast includes a frequency-switching unit which, in response to a monitored lamp current, instructs the dimming controller to change the frequency. Thus, the dimming control is made relying upon the lamp current being monitored. However, the dimming control may be insufficient for stably operating the lamp at different dimming levels due to a possible and transient fluctuation of the monitored lamp current. Thus, there is a demand for stably operating the lamp consistently at different dimming levels.